When structures are built in regions of permafrost, heat flow from the structure into the permafrost may cause thawing of the permafrost, degrading the stability thereof and resulting in structural damage. One approach to this problem is the use of passive thermosyphons to extract heat from the permafrost and maintain the same at a sufficiently low temperature to prevent thawing. A typical passive thermosyphon has an evaporator section buried in the permafrost, a condenser section exposed to the ambient and an intermediate section providing fluid communication between the evaporator and condenser sections. The thermosyphon is a closed system containing a working fluid such as CO.sub.2 or NH.sub.3. Such devices are substantially maintenance free and do not involve any operating costs.
A disadvantage of a passive thermosyphon is that it only operates when the ambient air temperature is below that of the soil. In permafrost regions this is typically from September to May. Consequently structural foundations may be vulnerable to unusually warm summers. Also, passive thermosyphons installed during summer months remain inoperative until fall, which may adversely impact construction schedules. Mechanical refrigeration systems with evaporator lines buried in the permafrost have been used to overcome these disadvantages of thermosyphons. While such mechanical systems do provide year around operation, they involve substantial operating costs and have the potential for an in-ground fluid spill if a buried circulation line fails.
It is a primary object of the present invention to provide a thermosyphon capable of operating in a self-contained passive mode when ambient temperature conditions are suitable or in an active mode with mechanical refrigeration assistance.
It is also an object of the present invention to provide a passive-active thermosyphon which allows for the use of mechanical refrigeration without the need for buried mechanical refrigeration circulation lines.
Other objects of the present invention include providing ground freezing for construction purposes and providing immobilization of toxic liquids. These objects may be accomplished in geographical areas having unfrozen or seasonally frozen ground.